I understand "casual" essentialy means it's OK to quit, while "high-commitment" means you are willing to take some responsability to avoid spoiling the fun for the rest of the players.

This doesn't mean casual games are for reckless players, because there are lots of different reasons that can lead to quitting (unreliable connection, tight work schedules, etc). For example: If I intend to spend the whole summer travelling I shouldn't join a high commitment game because come summer I'll abandon the game no matter what.
Further splitting of the hair goes beyond these two conventions, in my opinion, as gaming styles are a different subject altogether. After all part of the fun of any MP game lies in facing opponents with different mindsets.
I'm not criminalizing quitters either, **** happens in RL and players quit high commitment games too. The thing is: You sign up for a high commitment game if you expect to be available to play it to it's conclusion, and if you have to quit you do so in the least disruptive way for the other players. The same holds true for casual games, we should always avoid spoiling the fun for others no matter what the game tags are; but for casual games we accept more flexibility on that department (for players looking for quick gameplay, having low tolerance to defeat, or shifting schedules).
Dom3 is a complex, time-consuming game; some players are frustrated by the death of a game they have invested dozens of hours in, while other players are frustrated by long games with no real chances of winning. These two tags should sort one from the other. Commited players shift their objectives as the game develops (you cannot win this game? then focus on vengeance, or assist a player in a stronger position). The thing is: We should be honest with what we want out of the game beforehand.

All in all, this is a small enough community (Desura Dom3), we should be able to banish any controversy without disturbing the sleep of the elder gods in outdated threads.